1. Field of the Invention
The present invention relates to a catalyst composition for the synthesis of thin multi-walled carbon nanotube(MWCNT). More particularly, this invention relates to a multi-component metal catalyst composition comprising i) main catalyst of Co and Al, ii) inactive support of Mg and iii) optional co-catalyst at least one selected from Ni, Cr, Mn, Mo, W, Pb, Ti, Sn, or Cu. Further, the present invention affords thin multi-walled carbon nanotube having 5˜20 nm of diameter and 100˜10,000 of aspect ratio in a high yield.
2. Description of Prior Art
Carbon nanotube has a hexagonal honey comb shape in which one carbon atom is bonded with 3 adjacent carbon atoms. Further, the graphite plane is rolled in a round shape having nano size diameter. Specific physical properties are shown according to the size and shape of carbon nanotube. The weight of carbon nanotube is comparatively light due to its hollow structure. Further, the electrical conductivity is as good as that of copper, as well as the thermal conductivity is as good as that of diamond. Of course, the tensile strength is not less than that of iron. Carbon nanotube can be classified as single walled carbon nanotube, multi-walled carbon nanotube and rope carbon nanotube depending on its rolled shape.
Such carbon nanotube can be generally manufactured by an arc-discharge method, a laser vaporization method, a plasma enhanced chemical vapor deposition method, a thermal chemical vapor deposition method, a vapor phase growth method, or an electrolysis method. Among them, a thermal chemical vapor deposition method has been preferably used, because the growth of carbon nanotube can be made by the direct reaction between carbon source gas and metal catalyst without using the substrate. Further, high purity of carbon nanotube can be economically manufactured in a large amount according to a thermal chemical vapor deposition method.
In a thermal chemical vapor deposition method, the metal catalyst is necessarily required. Among the metals, Ni, Co, or Fe has been commonly used. Each particle of metal catalysts can act as seed for the formation of carbon nanotube. Therefore, the metal catalyst has been required to be formed as nano size particle. Of course, many researches for developing metal catalyst have been tried.
As a preparation method of metal catalyst reported until now, following preparation methods have been disclosed. First, the method comprising i) preparing the solution containing catalytic metals and support, ii) co-precipitating the catalyst composition by adjusting pH, temperature, and/or amount of ingredients, and iii) heat treating the precipitates under air or other gas atmosphere has been disclosed. Second, the method by drying or evaporating the suspension containing catalytic metal and fine grain support has been disclosed. Third, the method comprising i) ionizing the metal by mixing catalytic metal salt with cation particle support such as zeolite, and ii) reducing the ionized metal into metal particle by hydrogen or other reducing agent at high temperature has been disclosed. Fourth, the method by calcinating catalytic metal with solid oxide support material, such as, magnesia, alumina, and/or silica has been disclosed. Finally, the method of calcination for a metal composition has been disclosed where spray-drying of the catalytic metal precursor solution has been performed before calcination.
According to a catalytic chemical vapor deposition method, the metal catalytic components are slowly consumed in the process of synthesizing carbon nanotube. This consumption of metal catalytic components is caused by the inactivation of metal components by encapsulation, where carbon atoms encapsulate metal catalytic particles. Generally, re-activation of inactivated catalytic metal is neither possible, nor economical. In some cases, only few grams of carbon nanotube can be obtained using 1 gram of a metal catalyst composition including metal catalyst and support material. Therefore, the development of a highly active metal catalyst composition and of synthetic conditions has been required in order to produce the carbon nanotube in a commercially available scale
Following technologies have been reported in patent disclosures or references until now.
According to U.S. Pat. No. 5,165,909 by Hyperion Catalysis International Inc., a method for producing carbon fibrils which comprises i) calcinating a catalyst composition at about 500° C. under air atmosphere after Fe catalyst is supported by Al2O3, ii) reducing the catalyst composition using hydrogen gas at about 900° C., and iii) preparing carbon fibrils by reacting benzene as a carbon source under hydrogen atmosphere at about 1,000° C. has been disclosed. However, the catalytic yield for preparing carbon fibril is not so good. Further, the process for preparing metal catalyst requires complicate steps of calcination and reduction as well as more than 800° C. of high reaction temperature.
To overcome such drawbacks of above patent disclosure, U.S. Pat. No. 6,696,387 disclosed the catalyst composition comprising i) Fe as main catalyst, ii) alumina and/or magnesia particle as catalyst support and iii) at least one optional co-catalyst selected from V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, or the lanthanides. However, it is hard to obtain a precise multi-walled carbon nanotube with a high catalytic yield using this catalyst composition, because the uniformed dispersion between metal catalyst and support material cannot be accomplished due to the use of alumina and/or magnesia support material.
In PCT publication No. WO 2007/33438, a catalyst system for multi-walled carbon nanotube production has been disclosed. In this disclosure, a catalyst system for the selective conversion of hydrocarbons into multi-walled carbon nanotubes and hydrogen comprising a compound of the formula: (Ni, Co)FeyOz(Al2O3), has been disclosed. Further, as preferred catalyst compositions, CoFe2O4(Al2O3)4.5, CoFe2O4(Al2O3)16, and CoFe2O4 (Al2O3)32 have been disclosed. Therefore, the catalyst composition comprising i) (Ni, Co) and Fe as main catalyst and ii) alumina as catalyst support has been disclosed. However, it is also hard to obtain a precise multi-walled carbon nanotube with a high catalytic yield using this catalyst composition, because the uniformed dispersion between metal catalyst and support material cannot be accomplished due to the use of alumina support material.
To overcome the low catalytic yield caused by non-uniformed dispersion of a catalyst composition, the inventors of present application have already disclosed a catalyst composition in U.S. patent application Ser. No. 12/472,925 ‘Catalyst composition for the synthesis of thin multi-walled carbon nanotube and its manufacturing method’ filed on May 27, 2009.
In this U.S. patent application, a catalyst composition for producing carbon nanotube represented by following formula [Fea:Alb]x:My:Mgz has been disclosed. In this formula, Fe represents catalytic metal of iron, its oxide, or its derivative; Al represents catalytic metal of aluminum, its oxide, or its derivative; Mg represents inactive support of magnesium, its oxide, or its derivative; and M represents at least one transition metal selected from Co, Ni, Cr, Mn, Mo, W, V, Sn, or Cu, its oxide or its derivative.
Nevertheless, the inventors of present application have developed a highly uniformed and dispersed catalyst composition to enhance the catalytic yield by converting Fe into Co as main catalytic metal as well as by changing the ratio of Co and Al to accomplish the optimal molar fraction ratio. Further, all catalytic components containing inactive support material have been prepared using spray-drying, spray pyrolysis, or co-precipitation process in the aqueous solution.
Therefore, some drawbacks, such as, a low catalytic yield, pre-reduction of catalyst by using a hydrogen gas and non-uniformed dispersion of a catalyst composition have been solved by a novel catalyst composition and a preparation method of carbon nanotube using the same.
In the present application, the catalyst composition containing support material has been prepared using co-precipitation process in order to obtain a uniformly dispersed catalyst composition. Further, in the course of preparing a catalyst composition, a hydrogen reduction step has not been introduced. Finally, the inventors of present application have developed a multi-component metal catalyst composition comprising i) main catalyst of Co and Al, ii) inactive support of Mg and iii) optional co-catalyst at least one selected from Ni, Cr, Mn, Mo, W, Pb, Ti, Sn, or Cu. Further, the present invention affords thin multi-walled carbon nanotube having 5˜20 nm of diameter and 100˜10,000 of aspect ratio in a high yield.